Prior art power supplies typically fall into one of two classes. The first, "linear" power supplies, regulate an output parameter (current or voltage) by linearly changing a variable impedance in response to load conditions so as to maintain a fixed output. The second, "switched" power supplies, regulate an output parameter by controllably storing energy in a reactive component (typically an inductor) and discharging this energy to the load.
Linear regulators offer a number of advantages, one of which is good regulation. However, they also suffer from several drawbacks, the primary one of which is inefficiency. Power that is not consumed by the load is dissipated as heat. Linear regulators often have efficiencies below fifty percent.
Switching regulators also offer a number of advantages, a primary one of which is high efficiency--often above 85%. Also, switching regulators can provide "boost." That is, the DC output voltage may be greater than the DC input voltage. However, switching regulators also have a number of drawbacks. One is that they cannot provide precision (i.e. &lt;0.1%) regulation. Another is that the control circuitry used to dynamically control the switching characteristics (i.e. duty cycle) of a switching regulator to effect regulation in response to changing load conditions is generally complex, especially at high switching frequencies. Yet another is that the output signal usually includes some output ripple due to the switching operation. Finally, over-current protection is somewhat difficult to implement in many topologies.
In accordance with the present invention, a hybrid regulator is provided that incorporates both linear and switching-type circuits. While such a combination may at first be thought undesirable as simply compounding the flaws of one design with the flaws of the other, in fact the present design is well suited for certain applications.
In the preferred embodiment, the linear and switching circuits are cascaded. The switching circuit runs in open-loop fashion--without adaptive control. However, a feedback signal is derived from the output of the switching circuit and is used to control the linear regulator, effecting net control of the entire circuit. In this manner, a simple, low cost power supply is provided that provides simple over-current protection, characteristic of a linear supply, together with the boost capabilities of some switching topologies.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.